1. Field of the Invention
The invention relates to the field of the control of the operation of burner systems, especially safety control of jacketed radiant heating tubes, using a detector responding to the oxygen gas partial pressure in the exhaust gas.
2. State of the Art
In known systems, the air number is determined by the quantitative measurement of the oxygen partial pressure in the exhaust gas and, by controlling the oven or the feed of gas and air accordingly, the system is regulated at a given constant level. In this manner the quality of combustion can be monitored and kept constant. This known use of a detector responding to oxygen partial pressure in the exhaust gas as a means of measurement for the continuous regulation of combustion quality requires that the exhaust gas temperatures be within a relatively high temperature range of 500.degree. to 600.degree. C. At lower exhaust gas temperatures the sensor is suitable for quantitative measurement of the oxygen partial pressure only if it is heated accordingly.
In burner systems for industrial ovens, especially those intended for heat treatment in the steel industry, relatively low exhaust gas temperatures are desired. In recent times preference has been given to using the exhaust gas for preheating the combustion air. This is done, for example, in jacketed radiant heating tubes and recuperative burners. In combustion apparatus of this kind, continuous monitoring of the exhaust gas has not been performed heretofore. Exhaust gas analyzers are too expensive for such monitoring, and also they are slow-acting. The use of heated detectors in each burner is also too expensive. Without constant monitoring of the conditions of combustion, however, a transition from excess air to insufficient air, for example, goes undetected. The result is that, when the burner is operating under unfavorable combustion conditions, expensive repairs, such as the replacement of recuperative burners or jacketed radiant heating tubes or other successive recuperators, may become necessary, even if the burner system is given periodic maintenance and inspection. As it will be seen, the process of the invention permits a simple and constant monitoring of conditions of combustion even at low exhaust gas temperatures, e.g., in the use of recuperative burners or jacketed radiant heating tubes.
A special problem has been the leakage of a magnetic valve which serves for shutting off the gas feed line to a radiant heating tube. If a leak occurs in a magnetic gas valve, the as a rule unburned gas is let into the exhaust gas system. This cannot be tolerated. It has therefore been necessary for reasons of safety to provide double shut-off valves equipped with a blow-off means. Such an installation is extraordinarily expensive. The invention makes it possible, without additional instrumentation costs, to have continuous surveillance of the tightness with which a magnetic valve shuts off the gas feed line and to provide for an alarm to be given if leaks are detected. On the basis of the warning signal, a hand-operated shut-off valve can then be closed immediately.
Another trouble which can occur in the use of jacketed radiant heating tubes is a crack in the jacket. In this case the oxygen in the exhaust gas diffuses into the oven which is filled with inert gas. The consequence is oxidation of the material being heat treated in the oven. Heretofore it has been possible to locate the damaged radiant heating tube only by shutting off the system and individually pressure-testing the tubes. However, not only does oxygen escape through a crack in the jacket, but also, on account of the drop in the partial pressure, gases such as hydrogen, and hydrocarbons, also diffuse from without into the jacket and thus into the exhaust gas system. As it will be seen, this diffusion and possible inflow of inert and reactive gases into a jacketed radiant heating tube can be detected throughout the time of operation. When a defective jacketed radiant heating tube is detected, all of the connected media inputs and outputs are immediately shut off.